This invention is a method for treating spent nuclear fuel through the use of pyroprocessing.
There is an ongoing problem concerning the treatment of nuclear waste based on uranium oxide nuclear fuel. Currently, the oxide fuel is reduced chemically by lithium to a metal in a molten salt. This head-end reduction step, with the elimination of oxygen, precedes an electrometallurgical process. There are some difficulties in the application of the lithium head-end process when applied to chopped oxide fuel where massive pellets of oxide inside Zircaloy cladding must be attacked by the reductant. Among these are the inability of the lithium to penetrate the massive pellets in a reasonable time and consequently, the need to remove the cladding and reduce the size of the massive pellets. Another problem fuel is Thorium oxide, ThO2, clad with zirconium. Lithium will not reduce this oxide.
Applicants have found that massive solids of ThO2 and UO2 will dissolve in a select reagent of LiF-50 mol % ZrF4 (eutectic mp-500xc2x0 C.). As a result, the cited reagent is effective in treating spent nuclear fuel of the form UO2xe2x80x94ThO2xe2x80x94ZrO2 in a ceramic matrix and having a Zr cladding and other oxide residues containing Transuranic (TRU) Oxides.
Thus, the objective of this invention is to provide a reagent to chemically attack spent nuclear fuel in particular, but not limited to, fuel from light water reactors (LWR) or light water breeder reactors (LWBR).
Another objective is to combine the initial treatment of the spent nuclear fuel with plutonium recovery.
Additional advantages, objects and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.
To achieve the foregoing and other advantages, this invention is a method for treating spent nuclear fuel through the use of a pyroprocessing technique which is carried out in one vessel in a single molten salt bath. The chosen reagent is LiF-50 mol % ZrF4 (eutectic mp 500xc2x0 C.), but other ZrF4 rich salt could be used. The reagent is heated to a temperature between 550-650xc2x0 C. where is in a molten liquid state. The spent nuclear fuel is chopped into pieces and placed in a basket which is lowered into the molten salt. The products of the reaction between the salt and the spent nuclear fuel, in the form of UO2 and ThO2, are UF4, ThF4 and ZrO2. All of the products of the reaction dissolve in the liquid salt and thus, are effectively separated from the solids which remain in the basket. These solids include PuO2 and the noble fission product elements (Mo, Ru, Tc, Rh, Pd, etc). The liquid salt together with the dissolved constituents are subjected to hydrofluorination to convert the ZrO2 to H2O gas which is separated from the salt, and ZrF4 which remains with the salt bath. To recover the uranium product, the salt bath is then subject to fluorination yielding a gaseous UF6 product. The residual salt bath is then either recycled or converted to a fluorapatite waste. The Pu remaining in the basket can be recovered using a Pu recovery process.